Transistors are an essential component in modern mobile communications devices. Specifically, transistors play a vital role in the transmission and reception of radio frequency (RF) signals in the front end of a mobile communications device. Due to the decreasing form factor of mobile communications devices, the desire for a longer battery life, and support for an increasing number of stringent wireless communications standards, there is an ongoing need for smaller, more efficient transistor devices with improved performance characteristics.
As will be appreciated by those of ordinary skill in the art, one way to improve the performance at high frequencies (e.g., radio frequencies) is by using heterojunction bipolar transistors (HBTs). At high frequencies, HBTs offer many performance advantages over homojunction bipolar transistors. The performance advantages offered by conventional HBTs primarily arise due to a wider energy bandgap in the material of the emitter of the device as compared to the energy bandgap in the material of the base of the device. The wider energy bandgap of the emitter material allows for many parameters dictating the performance of the device to be optimized for high frequencies without degrading the current gain of the device.
For high power applications, multiple HBTs may be integrated into one semiconductor circuit to accommodate high currents as shown in FIG. 1. In some conditions, one of the HBTs, like HBT1, may run hotter than the other HBTs, and draw more current resulting in thermal runaway. One solution to provide good thermal stability for HBT1-HBTn is to apply emitter ballast resistors R1-Rn in series with HBT1-HBTn, respectively. These emitter ballast resistors R1-Rn may be provided by conventional thin film resistors, which may, however, significantly increase the size of the semiconductor circuit. Further, for some packaging technology, like flip-chip technology, emitters e1-en of the HBT1-HBTn will be connected together (not shown), which basically shorts out the emitter ballast resistors R1-Rn. As such, the HBT1-HBTn will not have different emitter ballast resistors to improve the thermal stability of the device.
Accordingly, there remains a need for improved semiconductor device designs to utilize the advantages of HBTs while achieving structure flexibilities such that HBTs may have different emitter ballast resistances for improved thermal stability performance. In addition, there is also a need to keep the size and cost of the final products effective.